This invention relates to an ink for use in ball point pens, and particularly in pressurized ball point pens, which is completely erasable for a period of several hours after application. The ink is safe to manufacture and store, and does not separate on standing.
Erasable ball point inks have been known for several years. Early efforts were directed to "liquid lead" pens using media such as a graphite suspension in a polyester resin as described in Cofield et al., U.S. Pat. No. 2,715,388. More recent approaches involve the use of a rubber or rubber-like vehicle dissolved in a volatile solvent which evaporates as the ink is spread upon paper. One of the earlier of such approaches is disclosed in Daugherty et al., U.S. Pat. No. 3,875,105.
In the Daugherty patent, a polyvinyl methyl ether resin was mixed with a volatilizing agent (toluene) and a high-boiling solvent (glycol phenyl ethers) for viscosity adjustment, as well as various other components. The mixture produced a writing medium comprising a heterogeneous liquid having a continuous phase containing the resin and volatile solvent and a discontinuous phase containing other materials of the formulation. Upon dispensing of the medium, the solvent volatilizes, and the polyvinyl methyl ether precipitates and captures colorant particles from the discontinuous phase.
A very similar system is described in Muller et al., U.S. Pat. No. 4,097,290. The Muller patent discloses the use of milled natural rubber (or its synthetic equivalent, cis-1,4 polyisoprene), or an epichlorohydrin elastomer, in a very low-boiling solvent such as petroleum naphtha or a volatile aromatic solvent. A dual solvent system is also disclosed.
The writing media disclosed in both the Muller and Daugherty patents appear not to be entirely homogeneous liquids, and tend to separate upon standing. When a pen containing these inks is not used for a period of time, a clear liquid drop appears at the pen tip, and must be wiped off prior to use. As a result of the phase separation that occurs at the ball, the first few characters written with the pen are faint and/or discontinuous. Commercial embodiments of some pens containing these inks carry a recommendation that the point be wiped with a tissue after long periods of non-use.
Furthermore, erasable inks of the prior art contain volatile flammable solvents. Since the ink manufacturing process always involves a lengthy mixing operation and frequently involves heating, the risk of fire or explosion must be considered. In addition, storage of a very large number of pens or large quantities of ink in a warehouse could create additional danger in the event of a fire.
It has been found in accordance with the present invention that an erasable ballpoint ink which is less flammable and less dangerous to handle than conventional inks and which does not separate, or "weep", can be produced from a particular combination of elastomer and low-boiling solvent. More particularly, it has been found that polyvinyl acetate or ethylene/vinyl acetate copolymers, when used in combination with trichloroethylene and other conventional ink components, provides an erasable ink which is less likely to smear and is less flammable than conventional inks, and is fully erasable for several hours; additionally, it produces an excellent trace immediately upon commencement of writing even if the pen has not been used for some time. The ink is removable with a soft pencil eraser.
Accordingly, it is an object of the invention to provide a ball point writing medium which is easily erased, and which consists essentially of a substantially homogeneous liquid which does not separate on standing. It is a further object of the invention to provide an ink which is safe and less flammable. These and other objects of the invention are effected by the compositions of the invention, specific embodiments of which are disclosed herein.